Many plants set more fruits than is desirable for the production of high quality fruit of commercial size. Plants setting too many fruits can lead to each fruit being smaller than if the fruit set is reduced, by a process called thinning.
Thinning of fruits, or the flowers that can become fruits, can be accomplished by hand removal of small fruits or flowers by hand thinning. Hand thinning is very costly and labor to complete the job can be difficult to obtain.
Chemical treatments have been used to commercially thin a variety of crops including apples and grapes. There are currently no effective chemical fruitlet thinners for stone fruit. Chemical thinners are sometimes not effective due to under thinning, over thinning, chemical burns due to phytotoxicity/, inhibition of fruit growth and abscission of leaves. For example, a chemical thinner that induces indiscriminate abscission of leaves and fruit would be of no commercial value. Consequently, there is a need for improved chemical thinning agents.
ABA is a naturally-occurring hormone found in all higher plants (Cutler and Krochko, 1999, Trends in Plant Science, 4:472-478; Finkelstein and Rock, 2002. The Arabidopsis Book. ASPB, Monona, Md., 1-52). Endogenous ABA is involved in a number of physiological processes including modulation of germination, dormancy, stomatal conductance, plant growth and leaf abscission (Milborrow, 1984, in Plant Physiology, ed Wilkins, 76-110; Kende and Zeevaart, 1997, Plant Cell, 9:1197-1210).
Quaghebeur (2005, US Patent Application No. 2005/0198896 A1) speculated that ABA causes defoliation, bloom inhibition, and fruit drop and induces hibernation-like states. The uses for ABA on fruit trees described in Quaghebeur (2005) are limited to enhancing leaf removal, reducing cherry cracking and reducing burgeoning growth caused by rain. When using ABA on apple and pear according to Quaghebeur (2005), quick leaf abortion is observed and airflow through the tree is improved. However, Quaghebeur (2005) does not mention the selective removal of flowers or young fruitlets preferentially over leaf defoliation and does not suggest the use of ABA as an effective thinning agent.
There are no published reports on the use of ABA as a thinning agent.
Our studies have shown that ABA applied at or shortly after bloom is an effective fruit thinner without causing unacceptable levels of leaf drop. Surprisingly, our studies have also shown that spraying fruit trees with ABA during bloom or shortly after bloom does not lead to fruit growth inhibition or a hibernation-like state, but rather to enhanced fruit growth compared to untreated trees.